Broadcasted overlay graphics are used for a wide variety of purposes. Among them, the most popular one is subtitling. Various methods exist to encode subtitles: text strings or compressed bitmaps. The number of colors used can vary by the type of application or the standard used. There is a growing need for transcoding between different audio/video standards: for archiving, transmitting or converting to a different physical media. However, different standards do not always offer the same features. In particular, in the context of subpicture elements, standards vary by the maximum color bit depth allowed.
Transmitted subtitles are quite often encoded using more bits than a destination standard can support. For example, a common broadcast standard uses 4 bits per pixel for subtitle, whereas a common storage standard uses only 2 bits per pixel. Another problem is that often destination colors cannot be chosen from a complete color space (usually, 3 components with range 0˜255). Instead, a fixed color palette is used, restricting possibilities for obtaining a good approximation of the original colors.
A DVD recorder can have a tuner capable of decoding a DVB broadcasted signal. The DVB program can include video, audio and subtitles. The DVB program is decoded, transcoded and then recorded on a DVD-VR disc. Focusing on the subtitling system, the Digital Video Broadcasting (DVB) standard for subtitling systems (ETSI EN 300 743 v1.2.1(2002-10)) allows subtitle displays with different regions. Each region can have different colors from each other region. For example, a single display can have up to 256 colors. Moreover, the colors in a particular display can be totally different from a previous display and a next display.
In contrast to the DVB standard, the DVD-VR (DVD Video Recording) standard (DVD specification for Rewritable/Re-recordable Discs, Part 3, VIDEO RECORDING, Version 1.0, September 1999) allows subpicture elements to have up to four colors, chosen from a sixteen color palette. Although the sixteen color palette can be selected from a complete color space, the sixteen color palette is fixed when recording begins.
It would be desirable to have an efficient, fast and reliable method of reducing a color space for transcoding subpicture elements to match criteria of a particular standard.